A capacitor is one of the essential passive elements for manufacture of a semiconductor integrated circuit (hereinafter abbreviated to an integrated circuit), particularly an analog circuit. Specifically, it is used as a storage capacitor in a pixel of a liquid crystal display device as well as an analog circuit such as a power supply circuit and a DRAM. Increase in the area of a capacitor leads to a higher cost of an integrated circuit, therefore, it is preferable that the capacitor occupy a small area but have a large capacitance value.
A gate insulating film of a transistor is advantageous in that it is thin and has a uniform thickness as compared to other insulating films used in an integrated circuit, has few defects or impurities, and has a high withstand voltage against dielectric breakdown in spite of its thickness. Therefore, the gate insulating film is suitable for a dielectric of a capacitor. In the case of an integrated circuit having both a transistor and a capacitor, MOS capacitance (gate capacitance) formed between an active layer and a gate electrode of the transistor is often used as the capacitor. A capacitor using MOS capacitance (hereinafter referred to as a MOS capacitor) can use as its dielectric a gate insulating film having the superior properties as set forth above, and it can be formed simultaneously with a transistor without increasing the number of the manufacturing steps.
The following Patent Document 1 discloses a structure of a conventional MOS capacitor.
[Patent Document 1]
Japanese Patent Laid-Open No. Hei 7-45789.